Changes in cell shape and movement of cytoplasmic bridges joining adjacent cells have been identified as the sources of the force that drives the morphogenetic process of inversion in Volvox. Conventional and high voltage transmission electron microscopy are being used to identify the cytological bases for cell shape changes and bridge migration. These methods will be supplemented by immunocytological methods employing antibodies directed against known cytoskeletal components. A number of mutant strains possessing genetic lesions of the inversion process are now in culture. These will be used to analyze the mecanism by which specific genes control such morphogenetic processes.